1. Field of the Invention
The invention relates to reducing power consumption in a portable communication device, and more particularly to a battery saving protocol for a portable computer or similar device.
2. Discussion of the Related Art
Portable radio modems and similar devices (e.g., pagers and cellular telephone modems) have become increasingly popular along with the portable computers and devices they support. Along with the popularity of the radio modems, the diversity of their uses has increased, and power consumption of the devices, although always important, has become increasingly critical to the popularity and utility of the devices.
The typical radio modem device (e.g., a pager using the BellSouth wireless data network and operating in accordance with the Mobitex Interface Specification (MIS), Revision 3A, September, 1994) includes a standby state or mode during which no messages are transmitted or received, and an operating state or mode during which communications between the portable radio modem and the base station are effectuated. Whenever the radio modem is to transmit a message it enters the operating state, awaits a frame signal transmitted by a base station and transmits to the base station in a slot that is chosen at random. The radio modem then stays in the operating state for a period of time to receive a message response from the base station. In keeping with the power consumption concerns, battery-saving protocols have evolved further dictating the operation of the portable radio modems in battery-saving modes.
The typical portable radio modem employs three different states of operation in a battery-saving mode: a standby state; a synchronized "on" state; and a quick-scanning state. In the standby state, only the time-keeping functions for synchronizing the radio modem to the base station are operational. In the background scanning state, a list of channels is scanned until a potential new base station is found.
In the operating state, the device scans channels under two conditions. If the device is currently synchronized with a given base station, it will periodically evaluate channels within a list specified by the network (current channel list) in order to facilitate a roam to a better base. If the device is not currently synchronized with a given base station, it will enter a quick-scanning state in order to find a base station to synchronize with--as quickly as possible. It is noted that roaming is the act of sending a packet to the network to indicate that the device has changed base stations. Roaming is not to be confused with the act of background scanning, as background scanning is the precursor to roaming.
In a typical network environment, transmission frames called sweep frames are transmitted periodically from the base. The sweep frames mark the start of a sweep cycle and contain network system parameters. As such, the sweep frames synchronize the radio modems with the network. A modem in battery-save mode synchronizes with a subtype 6 sweep frame (&lt;SVP6&gt;) broadcast by the network. The &lt;SVP6&gt; includes a traffic list and the timing parameters used in synchronization and message transactions for portable radio modems. The cyclical shift between the standby state and the operating state performed by portable radio modems is synchronized by the &lt;SVP6&gt; frames.
During the typical network operation, a radio modem accesses stored values of the current base and current system channels upon activation. If no current base has been stored, the radio modem immediately starts quick channel monitoring, using any system channel default lists. When a suitable base station is found, a roam/active signal is sent to the base station in order for the radio modem to identifying itself to the base station, and the portable radio modem then synchronizes with the &lt;SVP6&gt; frames of the selected base station. Once the radio modem enters the operating state, it remains there until it receives an &lt;SVP6&gt; frame containing a traffic list in which it is not included. If the radio modem has lost consecutive &lt;SVP6&gt; frames within a specified time, it stays in the operating state to resynchronize. If the radio modem has not succeeded in resynchronizing within a predetermined time, it begins quick scanning.
The roaming signal value, for the selected network channel, is typically derived by calculating an average value of the received signal strength (RSS) on the system channel during a specified time. While monitoring the current system channel when making the final decision before choosing a new base, the radio modem measures average received signal strength (RSS) during the reception of frame heads.
The roaming signal value of non-selected network channels is derived by calculating the average value of the received signal strength indicator (RSSI) during a predetermined number of periods. If the radio modem has lost consecutive &lt;SVP6&gt; frames within the time specified it stays in the operating state to resynchronize. If the radio modem has not succeeded in resynchronizing within the predetermined time it begins a quick-scanning sequence to acquire a channel. During the background scanning sequence if the modem finds a base with a roaming value higher than a good threshold base level, the radio modem will monitor that base exclusively for a network-defined period to monitor RSSI over frame heads. Following this period, if the roaming value acquired over frame heads is sufficient (good threshold), the device will initiate a roam to that base station--sending in a roam packet to the new base. Following successful transmission, the device will commence to operate and synchronize with the newly acquired base.
To complete the transfer of information back to the portable radio modem from a base station, down-link traffic to radio modems is indicated by a traffic list from the base station. When a radio modem receives a list containing one of its addresses, the radio modem remains in the operating state until transmission is completed or the process is interrupted due to a non-receipt of the &lt;SVP6&gt; frame. The radio modem once again will enter into its roaming sequence and attempt to reacquire a connection with a base station. Each change to a new channel requires the radio modem to expend power in order to transmit a signal to the base station, a signal required to re-identify the radio modem and indicate on which channel it is operating.
One disadvantage present in the typical battery saving modes and the corresponding network operations discussed herein is that the roaming sequence operations of the typical radio modem and the operations for acquisition of a new channel each expends a considerable amount of power. When a new channel is acquired or a new base station is acquired a transmission must be made, wherein a roam packet is sent to the selected base station to identify the radio modem to the base station. Needless changing of channels to acquire a slightly better signal so as to avoid dropped connections is not cost effective when weighed against the power consumption required to establish the connection. Consequently, there is a desire for an improved battery-saving protocol in portable radio modems and portable communication devices.